In mounting photographs, certificate and the like in a frame a mat made of cardboard or other suitable material is often used. In preparing such a mat it is necessary that the edges be trimmed precisely straight, and for the sake of appearance the inner edge of the mat is bevelled. Thus, it is desirable that the cutter be capable of cutting an edge that is perpendicular to the plane of the mat material and of also accurately cutting an edge bevelled at an appropriate angle. Cutting instruments heretofore available for this purpose have been cumbersome to use, relatively expensive to manufacture and difficult to adjust in the case of cutters having a capability of adjusting depth and angle of cut. Such a mat cutter is described in U.S. Pat. No. 2,924,010 and consists of a U-shaped body member having a smooth under surface which slides over the material to be cut. A shaft rotatably suspended between the two ends of the U have a slot therethrough which carries a cutting blade which projects downwardly to the surface of the work-piece. The shaft may be rotated to any desired angular position so as to establish a corresponding angle for the bevelled edge to be cut, and the blade may also be moved up or down to control the depth of cut. After the desired angular position and depth of the blade is selected the blade is tightened in the shaft, and the shaft locked to inhibit its rotation. The cutter is adapted to be guided along a straight edge by two guiding surfaces provided at the ends of the U. While the continuous adjustability of the angular position of the cutting blade might appear to be a desirable feature, the need for cutting bevelled edges at a variety of angles is rare, and the provision of this adjustability is not only costly but contributes to inconvenience in use as well as to a tendency for the cutting blade to shift from its pre-adjusted position during the cutting operation. The commercial implementation of the cutter described in this patent is formed of metal, and is thus unnecessarily heavy, and the upper surface of the body member is contoured to comfortably receive the heel and thumb of the right-hand of the user for moving the cutter along a straight edge; thus, it is very inconvenient for users who are left-handed.